Fluidization of finely divided solids forms the basis of many important industrial processes, the best known of which is fluid catalytic cracking. Fluidization seems to work best for solids in a particle size range from roughly 5 to 500 microns. There are, however, solids which lie within this particle size range which stick or cling together and cannot be reliably fluidized. These solids display a property known as cohesiveness. Neat cement is an example of such a material, although there are many other cohesive powders, such as grain dust, finely divided plastics, and more.
Fluidization of finely divided solids depends on the formation of gas bubbles which pass freely upwardly through a dense phase of particles totally supported by up-flowing vapor. Cohesive solids tend to form into masses which are bypassed by the vapor flow through cracks or fissures, rather than through bubbles. These fissures tend to become long-lived, so that the cohesive masses which develop are not subject to mixing. In many cases (for example, with neat cement), the solids fluidize very well for a short time after they are subjected to mechanical stirring. Once the cohesive masses form, however, it may be impossible to break them up by gas flow alone.
A well fluidized vessel can be used for blending solids which would otherwise need to be accomplished by repeated transfers of the solids from one vessel to another. Also, it is much easier to obtain a steady, controllable flow of solids from a fluidized vessel than from a nonfluidized vessel. Clearly, method and apparatus for fluidizing cohesive solids would be very desirable.